The invention relates to a battery management system (BMS) for a motor vehicle comprising at least one battery having multiple battery cells, an automotive electronic control unit comprising means of sensing battery quantities, wherein the battery quantities are the battery terminal current, the battery terminal voltage and the battery terminal temperature or the temperature within or in close proximity to the battery location, and an automotive alternator supplying a regulation voltage. In addition, the invention relates to a motor vehicle with a battery management system as well as to a method for charging the battery.
In the past decade there has been a move toward the electrification of automotive system functions allowing for the vehicle engine which would normally power these functions to be shut down when propulsion is not required. These include anti-idling functions, hotel loads and start stop systems. These systems place a greater reliance on the battery pack to act as a load leveling device. Traditionally battery packs are charged via a simple constant voltage. This minimal management of battery charging has functioned well for automobiles which only rely on the vehicle battery pack for starting, and thus have a very small depth of discharge. However in sleeper equipped trucks there are additional hotel and vehicle loads which lead to higher discharge depths. The constant voltage (CV) strategy falls short here, as the need to charge the vehicle quickly comes into conflict with the need to minimize float current to avoid overcharging and gassing once the battery is full. Because the control is static with regard to the battery state, one needs to decide between fully charging the battery with overcharging and avoiding overcharging by promoting undercharging and starvation.
In recent years there has been a switch to Absorbed Glass Matt (AGM) technology batteries which offer a higher C rating, higher design cycles and maintenance free operation. AGM batteries have greater charge acceptance, owing to their lower resistance and the ability to be repeatedly deeply discharged allowing for the powering of hotel load functions such as battery powered HVAC (Heating, Ventilation and Air Conditioning), CPAP (Continuous Positive Airway Pressure) machines and appliances. However, AGM batteries have a lower equilibrium voltage than flooded cell batteries which makes them subject to greater overcharge when using the same CV voltage set point. AGM batteries also receive greater damage due to overcharge then the legacy battery design due to a lack of excess electrolyte present. The loss of electrolyte due to overcharging is a contributing cause of multiple failure modes of the AGM battery. If they are used as a drop-in replacement for flooded batteries under a static fixed voltage charge regime, their service life will be a fraction of their intended design life.
While the vehicle engine is running the system voltage is regulated by the alternator. The system voltage also serves as the charging voltage for the vehicle battery pack; the static system voltage of 14.2 volts is a design trade off made to balance charge time for the vehicle's batteries against float current. A higher voltage will reduce charge times but increase float current where as a lower voltage will increase charge times but reduce float current. Ideally the voltage would be dynamic allowing for the faster charging when the battery is empty and little or no charging when the battery is full.
In today's vehicles the electrical charging, storage and starting systems together allow the vehicle to start. However, if the system is designed with no feedback of its state, failure comes without warning leading to stranded vehicles and expensive vehicle trouble shooting.
The following paragraphs detail the shortcomings of the existing static battery control.
One of the shortcomings of the existing static battery control is the overcharging of batteries. The overcharging of batteries has been shown by engineering tear down analysis to be a predominant cause of early failure for AGM batteries installed in vehicles with a fixed voltage charging strategy. Overcharging of a battery pack occurs when a fully charged pack continues to be charged. In a CV charger this occurs when the battery has an equilibrium voltage which is substantially lower than the vehicle system voltage. Equilibrium voltage varies with temperature and State of Charge (SoC) and it moves lower with high ambient temperature. Liberation of hydrogen gas from the negative plate and oxygen from the positive plate is seen during the charging process. However, when a fully charged battery continues to be charged, due to a float voltage that is too high, thus causing high float current, the energy imparted to the battery causes high rates of gassing. Recombination of these gases is an exothermic process. Internal heating due to recombination drives cell equilibrium voltage lower which increases float current and gassing still further. If the heating caused by excessive float charging exceeds the batteries' ability to dissipate this heat for a long enough duration, the battery enters a condition know as “Thermal Runaway”. This positive feedback phenomenon when allowed to continue will cause catastrophic failure of the battery. Failure of one battery within the pack increases reliance on remaining batteries which in turn shortens their life. Short of this catastrophic failure, excessive gassing will exceed the batteries ability to recombine the liberated gases. Internal pressure built by plate gassing will eventually exceed the threshold of the cell's pressure regulation valve. Once this valve opens, “burping” the excessive pressure to the atmosphere, the escaped gases will never be reclaimed. This loss of electrolyte leads to dry out of the fabric separator which is a contributing cause to several failure modes.
Another shortcoming of the existing static battery control is the undercharging of the battery pack. Undercharging of the pack occurs when the energy consumed by loads when the engine is off is greater than the energy that is stored in the battery by the vehicle's alternator during the entirety of the next drive cycle. This phenomenon is referred to as “starvation”. Under a static charge strategy the single system voltage of 14.2 V serves as a compromise between minimizing charge time and minimizing float voltage. This compromise ensures longer charge times than what is possible given the charge acceptance of the batteries and the capacity of the alternator.
An additional shortcoming of the existing static battery control is the over discharge of the battery. Cycles seen during the life of the battery is known to correlate strongly to depth of discharge. When selecting a battery for a given application, it will last for a greater amount of cycles if a larger battery is selected as all else being equal the depth of discharge for a larger battery will be smaller. The lead within a battery is mechanically active; when the battery discharges the formation of lead sulphate causes the plates to expand. During charge the lead sulphate crystals are returned to solution causing the plates to contract. Higher DOD is associated with greater mechanical expansion and contraction of the plates which reduces the adherence of the active material to the grid. This is referred to as a “softening” of the active material. Over discharge is currently managed by “shedding” loads when the battery reaches a low SoC. However in current vehicles the determination of low SoC is made through voltage measurements which are known to correlate to SoC with an error of +/−20%. This inaccurate understanding of SoC impedes the system's ability to balance battery calendar life against run time utility as the most conservative load shed points must be chosen to account for the high degree of uncertainty in the SoC voltage measurement. A more accurate understanding of SoC coupled with an assessment of the system loads and use case yields the ability for the system to advise the user when and how long to run the engine of the vehicle, raising the SoC of the battery to a high enough level to allow the battery to last through the user's sleep cycle thus allowing for a more well rested user.
Another disadvantage results from charging the battery outside the State of Function (SoF) envelope. The lead acid battery is a time variant electrochemical system. There are many factors which determine the ability to accept or provide charge at any given time. The amount of charge acceptance which the battery is capable of varies with cell temperature and SoC. At the extremes of temperature it is necessary to limit the current charged to the battery in order to prevent battery damage. In heavy duty vehicles there exist short periods of time where the battery cell temperature reaches these extremes. With system knowledge of the battery's state of function the system could delay charging and prevent battery damage. One such avoidable extreme is cold winter operation where charging could be delayed until engine heat brings the battery into a chargeable range. Another avoidable extreme is hot summer “grade pulls” and idling periods where engine heat and lack of air movement can drive battery box and eventually internal battery temperature past the temperature where the battery can be charged without risking vigorous gassing, dry out and possible thermal runaway. The lack of an accurate understanding of SoC, State of Health (SoH) and temperature means the system designer cannot design the system to only charge within the batteries' SoF. Use outside of SoF is by definition damaging and shortens battery life.
SoC and SoH are internal battery state quantities. Those internal battery state quantities are by definition “latent” and thus unable to be measured via an external sensor. Estimation of SoC is known from prior art. US 2013/0138370 A1 provides a battery SoC estimation method and battery management system. Estimation of SoH is known from prior art as well. US 2010/0244846 A1 describes a method for determining the SoH of a battery.
An additional failure mode of lead acid batteries is sulfation. Sulfation occurs as an integral part of the battery discharge chemical reaction which converts lead to lead sulfate. However when the pack is kept at a low state of charge for long periods of time or it is operated at a partial state of charge, seldom achieving 100% SoC, the smaller lead sulfate crystals on the surface of the negative plates are allowed to aggregate into larger crystals or so called “hard sulfation”. It is known that prescribed overcharge, also known as an equalization charge stage, can be employed to break down hard sulfation thus winning back battery electric charge capacity and improving overall pack life. An equalization charge phase is by definition an overcharge of the battery and thus must be both used sparingly and closely monitored to prevent electrolyte dry out and thermal runaway. The lack of an understanding of the individual use case of the battery as well as the lack of ability to precisely control the charge voltage prevents vehicles with a static charge strategy from mitigating sulfation through equalization.
A contributing factor to battery aging and capacity loss is lack of cell balance. An automotive starting and hotel load battery consists of multiple battery cells connected into a series string. The string is connected to the vehicle and other batteries at the negative end of the bottom cell in the string and the positive end of the top of the string. According to Kirchhoff's law the current passing through each series connected cell is equal. If the charging and discharging efficiencies were equal for each of the cells, the change in SoC due to the equal current would also be equal. Batteries used in the automotive environment, however, differ from this ideal. The series string of cells can exhibit different charge/discharge efficiencies due to many reasons including, differences in internal cell temperature due to differing orientation of the cell, manufacturing variance related to the amount of electrolyte present in each cell and varying amounts of current consumed by cell reactions other than the primary charging reaction such as reactions involved with the closed oxygen cycle.
There are several known scenarios in which the shedding of load occurs but high pack current draw persists. In one scenario a load has been placed between the battery and the load shedding switch. In this scenario even with all the loads shed by the automatic system the load still draws upon the battery. In a second scenario the battery continues to be discharged by a load which is necessary for safety, such as exterior lighting. In both these scenarios ideally the most probable source of the load could be estimated by examining the vehicle serial communication bus information. Information such as the DoD and length of time at DoD, otherwise known as load profile, could be relayed to a diagnostics ECU (Electronic Control Unit) which in turn relays the information to a back end telematics server. This information then forms a behavioural loop between the driver, the vehicle and the fleet's management. This additional information about the use patterns for the vehicle's batteries and charging system, also known as powernet function, allows the trucking fleets to encourage proper use scenarios and discourage improper use scenarios.
Methods of charging are known from prior art in order to avoid the failings described above. For example, there is prior art for varying system charging voltage with temperature, known as float voltage compensation.
It is the object of the present invention to provide a reliable and competitive battery management system to enlarge battery life time and to improve failure predictability.
According to the invention, this object is solved by a battery management system, a vehicle as well as a method for charging a battery having the features according to the respective independent claims. Advantageous implementations of the invention are the subject matter of the dependent claims, of the description and of the figures.
The battery management system (BMS) according to the invention can be disposed in a motor vehicle. The battery management system comprises at least one battery having multiple battery cells, an automotive electronic control unit comprising means of sensing battery quantities, wherein the battery quantities are the battery terminal current, the battery terminal voltage and the battery terminal temperature or the temperature of air within or in close proximity to the battery location, and an automotive alternator supplying a regulation voltage. The automotive electronic control unit is adapted to drive the battery through a charging cycle, in which a certain charge voltage and/or a certain charge current is supplied to the battery in order to charge the battery, and the automotive electronic control unit is adapted to control the regulation voltage based upon the sensed battery quantities in order to supply the certain charge voltage and/or the certain charge current to the battery.
Means of sensing current and current sensors, respectively, can be at least one of the following: a copper current shunt which is temperature compensated in firmware, a copper current shunt which is temperature compensated by the addition of a second metal, an analog-to-digital converter measuring the difference in voltage at opposite ends of the current shunt or a Hall Effect sensor. The means of current sensing can be multiplexed across a plurality of conductive paths to be sensed. The multiplexing of a plurality of bus bar shunts allows a plurality of batteries to be managed within one or more packs. In a preferred embodiment there are four parallel connected sensors however it could also be any other number of sensors corresponding to the battery pack(s) installed on the vehicle. Means of sensing temperature and temperature sensors, respectively, can be at least one of the following: a thermistor, a resistive temperature detector, a band-gap, thermopile, IR imaging, or a thermocouple.
There are several aspects of battery sensor design which enhance its accuracy and reduce its cost. The use of a highly accurate data converter allows for a shunt resistance that is half that of standard designs. This low shunt resistance is favourable in minimizing overall resistance of the vehicle charging/starting circuit, allowing for more power to be delivered to the starter during the engine crank event. Classical low pass filters will reduce both noise and signal leading to an unacceptable loss of fidelity which will impede overall system accuracy. An additional aspect of the invention includes the use of a non-linear diffusion filter to remove noise while retaining a greater amount of the signal allowing for an increase in sensor and overall system fidelity.
According to the invention the battery management system controls the charging of the battery as well as, in some configurations, the load shed functions of the vehicle electrical system to increase battery life and predictability of battery pack remaining useful life. The remaining useful life is an estimate of the average time to failure for the battery being considered. Charging is controlled by varying commanded system regulation voltage of the vehicle alternator based on current, voltage and temperature sensors attached to the battery as well as expert information obtained through laboratory testing of the battery being managed. In other words, charge control of the battery pack is achieved by the changes in the commanded alternator regulator voltage set point which affect changes in the voltage seen at or the current flowing through the battery. Those changes in the regulation voltage cause changes of the amperage charged to the battery. The regulation voltage and thereby the amperage charged to the battery are changed through changes according to the resulting voltage and temperature changes sensed by the sensing means. Within this dynamic battery control the battery is driven through the charging cycle. A battery pack driven through a charging cycle according to the invention offers the user the greatest utility while also retaining the greatest capacity. Battery life is extended by avoiding both overcharging and undercharging in an environment with particularly variable temperature. Thus an optimal use of the battery pack can be achieved. For heavy automotive applications the control of charging represents the greatest opportunity for extending battery life. The additional battery life offers a compelling value proposition for the end user as well as the OEM (Original Equipment Manufacturer). The incremental cost of the preferred embodiment added to the cost of vehicle manufacture can be many times less than the reduction in total cost of ownership realized by the addition of the invention.
In a preferred embodiment, the automotive electronic control unit is adapted to provide a first stage within the charging cycle, wherein the battery is charged with a constant charge current with a first amperage to a first state of charge (SoC). Generally, the SoC is defined as the ratio of the stored charge currently available relative to the charge available after a full charge of a certain battery. It is a relative value based on the certain battery at a certain age. SoC is an internal (or latent) state of the battery and is not observable from the outside. The first stage can be denominated as a bulk charging phase, e.g. spanning 0-80% of the SoC. The alternator regulation voltage can be controlled in order to deliver a bulk charging stage in which a constant current is delivered to the battery. In this phase the battery exhibits a high charge acceptance. Thus, the charge current can be maximized at a low state of charge. This decreases charge time which in turn decreases undercharge if the charge sequence is interrupted by the driver shutting down the vehicle engine. The decision to end this stage can be based on SoC estimation or alternately on battery voltage rising past a predetermined threshold. Within this bulk stage, one or more current magnitudes may be used. When the battery voltage resulting from the constant current charge rises past a predetermined threshold a new constant current value approximately half the magnitude of it's predecessor is used. In a preferred embodiment the number of different magnitudes used is four. This number achieves a balance between fast charging, which is advantageous, and heat generation which is known to be damaging. The values used for these constant current magnitudes can be advantageously adjusted with the estimated internal battery temperature to optimize the competing needs of charge time and primary to parasitic charge ratio with the need to not excessively raise the temperature of the battery through heat internally generated through the charging process.
Advantageously, the automotive electronic control unit is adapted to provide a second stage within the charging cycle, wherein the battery is charged with a constant charge voltage to a second state of charge, while monitoring that the battery current stays within a predefined limit. The second stage can be denominated as an absorption phase, e.g. beginning at 80% of the SoC. The alternator regulation voltage is controlled in order to deliver an absorption charging stage in which a constant voltage is delivered to the batteries while monitoring that the battery current stays within predefined, acceptable limits. Generally, in this phase the battery charge acceptance declines. Thus, a relatively high charge voltage is chosen which remains below gassing voltage for the current cell temperature. Absorption mode is controlled by a timer whose duration is proportional to the time duration of the bulk phase and a lower threshold of accepted charge. When the absorption time expires or the battery accepted charge falls below a pre-determined threshold the absorption period ends. Alternately the end of the absorption stage can be based on SoC estimation. As a further advantage of this stage the constant charge voltage of the absorption stage can be adjusted downward with increasing battery temperature. Higher charge voltages have the advantage of creating a better ratio between primary and parasitic reactions within the battery, however when the voltage is too high relative to the battery equilibrium voltage there will be excessive gassing as the plates. Temperature compensation of the charge voltage seeks to balance these competing needs.
Preferably, the automotive electronic control unit is adapted to provide a third optional stage within the charging cycle, wherein the battery is charged with a constant charge current with a second amperage in order to equalize different states of charge of the battery cells. The third stage can be denominated as an equalization phase. The alternator regulation voltage is controlled in order to deliver an equalization charging stage in which a small constant current is delivered to the battery to make cell SoC more equal and reduce sulfation build up upon the negative plate. In other words, the SoC of the battery cells of the battery are balanced. Occasionally the batteries are equalized by charging the battery with a small current until system voltage rises to a certain value allowing for cell equalization and a breakdown of sulfation. Sulfation is optimized against grid corrosion and battery life is extended. It is a further advantage of the design to adjust downward the duration of the stage and the magnitude of current used with increasing temperature. This process is referred to as temperature compensation.
It proves advantageous if the automotive electronic control unit is adapted to provide a fourth stage within the charging cycle, wherein the battery is charged with a charge current which is dependent on the sensed temperature of the battery in order to maintain the second state of charge. The fourth stage can be denominated as a float phase. In the float phase the charge current is set to a level which only replaces what is lost to self-discharge. In this phase system voltage is varied to ensure that the battery current stays at this value. The current charged to the full battery is regulated so that it balances the replacement of self-discharged energy but does not overcharge. In a different embodiment of the invention the fourth stage controls the battery current to zero until such time as the SoC falls below a threshold at which point a new charging cycle is initiated.
In a further embodiment of the invention the battery is charged using two different charge strategies which are interchanged based on determination of system need and or predetermined ratio.
In a preferred embodiment, the first charge strategy is a constant current strategy and the second strategy is a current interrupt charge strategy. The constant current charge strategy is controlled such that the ending SoC of the battery is above 90% but less than 100%, this ensures no overcharge is seen with this method. If this was the only strategy used the battery would soon fail due to undercharging. However if a second method is employed which charges the battery past 100% SoC it imparts overcharge that reverses the decrease in capacity seen during successive charges under the first charge strategy. Dividing the overcharge seen in the second charge strategy by the number of times the first charge strategy was used plus one for the overcharging strategy itself leads to a very low percentage of overcharge per cycle. This method of minimizing both overcharge and undercharge has been shown to increase the total number of cycles performed by the battery prior to failure.
In the first charge strategy of the above two strategy embodiment the automotive electronic control unit (ECU) is adapted to provide a first stage within the charging cycle, wherein the battery is charged with a constant charge current with a first amperage until battery voltage raises to a first battery voltage. At the point of exceeding this first voltage, the ECU initiates a second stage of the constant current charge strategy where a second constant charge current is imparted to the battery at around half the magnitude of the first. This process of halving the current for the successive stage continues until a high state of charge is achieved. Constant current charge regimes can be realized with an arbitrary number of stages, however in a preferred embodiment the optimal number of stages is four. Chargers with less than four stages have longer charge times and a higher amount of heat generation. The final stage of the constant current charge strategy has the smallest magnitude of current at lower charge currents and thus lower charge voltages a greater percentage of the charge current contributes to parasitic side reactions resulting in heat generation. A greater amount of stages allows for less charge time to be expended in the final stage for a given target SoC. This effect of reducing the extent of side reactions and decreasing charge time shows diminishing returns above four stages.
In a preferred embodiment, the automotive electronic control unit is adapted to estimate an internal battery state through use of an equivalent circuit model. The equivalent circuit model is a lumped electrical circuit which approximates the complex electrochemical reactions within the battery with a simple electrical analogy. In order to allow the model to retain accuracy over changes in system load, SoC, and temperature, the embodiment includes a look up table for the values of the lumped circuit elements. The equivalent circuit values are found by recording the voltage response to a current pulse over ranges of battery current, SoC and temperature. The model lumped circuit element values are then varied until the model voltage response most closely matches that which was previously recorded in a battery lab using a physical battery. This process is repeated until a look up table with sufficient resolution to capture the dynamics of the system is assembled. Within the ECU this look up table is consulted at each time step of the model to employ the optimal equivalent circuit values for the system conditions during the time step.
Internal battery state is defined as an understanding of the SoC, SoH, SoF and Remaining Useful Life (RUL) of the concerned battery. The estimation is built upon values obtained from measuring voltage, current and temperature of the battery. These sensors are identical to those needed for the charger and thus can be shared. This internal state of the battery is relayed to the power net controlling ECU(s) to base shedding of hotel loads on battery SoC rather than less accurate voltage measurements. Information regarding battery use is relayed to the vehicle diagnostic systems to be forwarded to the fleet management professional as part of a remote diagnostics regime.
SoH is a relative measure of the capacity of the battery. It's defined as the 100% SoC capacity of the battery at the current point in time over its 100% SoC capacity on the date of manufacture. Due to sulfation, grid corrosion and expansion, and dryout a slow degradation of capacity and increase of internal resistance are experienced over the life of the battery.
SoF is the instantaneous ability of the battery to accept or provide current from/to the alternator or load. These limits dictate the control decisions required to properly manage the battery. The closer control decisions are to the actual SoF the longer the battery life, all else being equal. SoF is estimated as a function of SoC, SoH and temperature.
The equivalent circuit model can be calibrated or adjusted at points in time where the rested voltage can be measured or estimated allowing the look up table to be referenced to obtain the true state of charge of the battery or battery pack. The state of charge of a battery can be calibrated by applying a pulsed current forcing function to the battery whose voltage response is examined in light of the previously characterized function to yield SoC.
Applying a pulsed current forcing function provides one means to yield the capacity, State of Health and Remaining Useful Life (RUL). It can be provided that the estimated RUL and/or the SoC, particularly a low SoC, are displayed to a driver of the vehicle and/or maintenance personnel and/or a fleet manager. Particularly in a sleeper truck, it is possible to alert the user to the need to charge the vehicle batteries before the user goes to sleep such that the charge stored in the batteries is sufficient to power the anticipated load throughout the sleep cycle of the driver. In order to accomplish this, a running average of overnight demand is stored within the device. An average is also drawn of SoC and SoH during the overnight segment. This information yields the total charge available to drive loads through the evening. From both the anticipated load and the anticipated capacity to drive load a shortfall can be made up by the driver charging the vehicle batteries prior to the overnight segment. This advisement is made through the vehicle cluster as a user alert or as information that can be queried though the vehicle cluster menu.
By means of providing the equivalent circuit model the internal battery state can be estimated to properly schedule maintenance charges.
It can be provided that the battery management system has an analogue voltage sense line and/or a serial data bus for controlling the regulation voltage of the vehicle alternator.
A motor vehicle according to the invention comprises a battery management system. Particularly the motor vehicle can be a heavy duty vehicle like a sleeper compartment equipped truck.
In addition, the invention relates to a method for charging the battery having more than one battery cells by driving the battery through a charging cycle. In a first stage of the charging cycle the battery is charged with a constant charge current with a first amperage to a first sate of charge (SoC), in a second stage of the charging cycle the battery is charged with a constant charge voltage to a second state of charge, while monitoring that a battery current stays within a predefined limit, in a third stage of the charging cycle the battery is charged with a constant charge current with a second amperage in order to equalize different states of charge of the battery cells, and in a fourth stage of the charging cycle the battery is charged with a charge current which is dependent on a sensed temperature of the battery in order to maintain the second state of charge.
The preferred embodiments presented with respect to the battery management system according to the invention and the advantages thereof correspondingly apply to the motor vehicle according to the invention as well as to the method according to the invention.
Further features of the invention are apparent from the claims, the figures and the description of figures. All of the features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of figures and/or shown in the figures alone are usable not only in the respectively specified combination, but also in other combinations or else alone.
Now, the invention is explained in more detail based on a preferred embodiment as well as with reference to the attached drawings.